When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Sunday, December 21, 2014

Peter Gleick: The answer on human-caused climate change is in

The three-year California drought isn’t bad just because of a severe shortage of rain and snow; previous droughts have seen less precipitation. It’s bad because the past three years have also been by far the hottest in the 119-year instrumental record.JAE C. HONGTHE AP

As our planet warms, scientists and the general public are increasingly asking if human-caused climate change influences the extreme weather events we see all around us. Until recently, the answer was always “we don’t know yet.” Today, the answer is increasingly “yes.”

Earth is in a remarkable transition from a world in which human influence on climate has been negligible to one in which our influence is increasingly dominant. One of the most active research areas in the climate sciences is the field of detection and attribution: the effort to see and identify the fingerprint of climate change in our extremes of weather.

This is tough because the day-to-day fluctuations in weather are naturally large or “noisy.” But scientists have long known that as climate change worsens, we would eventually reach the point when the signal of human influence would rise above and become distinguishable from the noise of natural variability. The California drought is a case in point.

For the past three years, California has been experiencing a bad drought, most simply defined as the mismatch between the amounts of water nature provides and the amounts of water that humans and the environment demand. The state, like any other region of the world, experiences extreme hydrologic events naturally, including floods and droughts. Reconstructions of ancient climates from tree rings, ice cores, pollen records and other paleoclimatic assessments reveal extensive and persistent droughts in the past.

Is this a signal of climate change? Recently, there has been a growing debate about the links between climate change and the drought. The debate stems from confusion over two very different questions:

▪ Is California’s severe drought caused by climate change?

▪ Is California’s drought, no matter the cause, influenced or affected by climate change?

These questions are not the same thing. Yet the media, the public and even some scientists have mixed them up. A popular analogy is that we cannot know whether any particular home run hit by a baseball player pumped up on steroids was caused by the drugs. Ballplayers hit home runs naturally. But we know that these drugs influence and distort the average performance of the ballplayer over time.

The influence of climate change on extreme events works the same way: Science cannot at the moment tell us much about whether climate changes have caused any particular extreme event. But scientists from many different fields now see parallel lines of evidence from a combination of basic climate science, models and real-world observations that human-caused climate change is already influencing and worsening the risks of extremes such as heat waves, droughts and floods.

For the California drought, this evidence includes the fingerprints of higher temperatures and changes in atmospheric circulation patterns in a climate pumped up by greenhouse gases emitted by human activities. These higher temperatures have led to extra drying of soils, greater agricultural water demand, earlier and faster snowmelt, and higher evaporative losses in reservoirs.

California is not alone in experiencing the growing consequences of climate change. Evidence that climate change is influencing extreme events all over the world is now pouring in, including heat waves in Europe, coastal damage in the Eastern U.S. during extreme tides and storms, flooding in the United Kingdom from more intense precipitation events, drastic loss of Arctic ice, and droughts in Australia and the southwestern U.S.

The rainy season in California has started again, ironically with another set of extreme events in the form of intense storms. Many of us hope that the state will see an average or even a wet year to refill dangerously overpumped aquifers and depleted reservoirs. But if there is any lesson to be learned from the past few years, it is that California and the rest of the world must prepare for a future where the influence of human-induced climate change is ever more apparent. And costly.

Peter H. Gleick is president of the Pacific Institute, an environmental policy group based in Oakland.